java中try-with-resources自动关闭io流

0 简要说明

在传统的输入输出流处理中，我们一般使用的结构如下所示，使用try - catch - finally结构捕获相关异常，最后不管是否有异常，我们都将流进行关闭处理

  try {//业务代码} catch (SQLException e) {// 异常，数据回滚try {connection.rollback();} catch (SQLException ex) {log.error(ERROR_MSG, e);throw new IllegalArgumentException(e);}log.error(ERROR_MSG, e);throw new IllegalArgumentException(e);} finally {if (Objects.nonNull(preparedStatement)) {try {preparedStatement.close();} catch (SQLException e) {log.error(ERROR_MSG, e);throw new IllegalArgumentException(e);}}if (Objects.nonNull(connection)) {try {connection.close();} catch (SQLException e) {log.error(ERROR_MSG, e);throw new IllegalArgumentException(e);}}}

try-with-resources

在jdk1.7之后，推荐使用try() {} catch(IOException e){}的方式来处理io流，它可以自动关闭流。如下所示，是一个简单的按行读取文件内容的示例

 @Testpublic void readFileContent() {File file = new File("d://DumpStacks.log");try (BufferedReader reader = new BufferedReader(new FileReader(file))) {String line;while ((line = reader.readLine()) != null) {log.info("line : 【{}】" , line);}} catch (IOException e) {log.error("error msg : 【{}】", e);throw new RuntimeException(e);}}

抛异常情况

java.lang.RuntimeException: java.io.FileNotFoundException: d:\DumpStacks.log (系统找不到指定的文件。)

at com.geekmice.springbootselfexercise.NoDaoTest.readFileContent(NoDaoTest.java:347)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)

Caused by: java.io.FileNotFoundException: d:\DumpStacks.log (系统找不到指定的文件。)
at java.io.FileInputStream.open0(Native Method)
at java.io.FileInputStream.open(FileInputStream.java:195)
at java.io.FileInputStream.(FileInputStream.java:138)
at java.io.FileReader.(FileReader.java:72)
at com.geekmice.springbootselfexercise.NoDaoTest.readFileContent(NoDaoTest.java:340)

反编译之后可以看到自动关闭流操作

打开target，找到对应字节码文件

 @Testpublic void readFileContent() {File file = new File("d://DumpStascks.log");try {BufferedReader reader = new BufferedReader(new FileReader(file));Throwable var3 = null;try {String line;try {while((line = reader.readLine()) != null) {log.info("line : 【{}】", line);}} catch (Throwable var13) {var3 = var13;throw var13;}} finally {if (reader != null) {if (var3 != null) {try {reader.close();} catch (Throwable var12) {var3.addSuppressed(var12);}} else {reader.close();}}}} catch (IOException var15) {log.error("error msg : 【{}】", var15);throw new RuntimeException(var15);}}

当使用了try(){}代码块之后，代码执行完毕，就可以进行流的自动关闭，比传统的方式简洁了不少。
try()这部分，可以有多个语句，语句之间分号隔开，也可以同时包含输入流和输出流。最后执行完毕，统一关闭。